Vacuum pump



Jan. 29; 1963 D. STEVENSON VACUUM PUMP iled Nov. 20, 1961 IN VEN TOR. LSTEVENSON DONALD Emma 5/ 6/144...

ATTORNEY nitd rates 3,675,689 VAQUUM I UMP Donald L. Stevenson, Albion,N.Y., assiguor to Consolidated "acuum Corporation, a corporation of NewYork Filed Nov. 26, 1961, Ser. No. 153,544 4 Slaims. (Cl. 230-161) Thepresent invention relates to vapor vacuum pumps and, more particularlyto difiusion and ejector pumps of the type which utilize a jet nozzleassembly comprising one or more jet nozzles.

Generally, the use of diffusion and ejector pumps for creating vacuuminvolves the very disagreeable phenomenon of backstrearning which occurswhen molecules of oil vapor migrate back into the vessel to beevacuated. Various measures have been taken to counter this movement ofoil vapor, most notably, the use of highly cooled baflles and the likewhich are positioned between the vacuum pump and the vessel to beevacuated. The disadvantages in using baflles is that the conductancetherethrough is seriously limited and this in turn greatly reduces theeflective pumping speed of the pump and baflle combination. Generally,the amount of backstreaming through a bafile of conventional design isinversely proportional to the impedance of the baflle,- that is, themore the baflle is capable of reducing the backstreamin the lessconductance there is through the baflle. i

Other attempts to minimize backstreaming have involved the use of a highconductance baflle and so-called cold caps on each of the nozzles fromwhich the vapor jet issues, more notably, the top jet nozzle. These coldcaps are cooled by various devices such as Peltier couples or coolingcoils coiled around the cap and connected to an outside source ofcooling medium. The cooling of these caps requires equipment that isrelatively expensive and conduits connecting the cooling devices toequipment located outside the pump extends through the wall of the pumphousing thus offering an increased number of places where leaksmaydevelop.

It is the principal object of the present invention to minimizebackstreaming in a vapor type vacuum pump utilizing a minimum of partsand at relatively low cost.

Other objects and advantages will become apparent after studying thefollowing description and claims taken in conjunction with theaccompanying drawings wherein:

FIGURE 1 is vertical section through a diffusion vacuum pump showing theassociation of the present invention with the pump;

FIGURE 2 is a fragmentary view, partly in section, of the top jet nozzleof the pump of FIGURE 1, showing the present invention applied thereto;

FIGURE 3 is an enlarged fragmentary view of another jet nozzle of thepump showing the present invention applied thereto.

Referring more particularly to FIGURE 1 of the drawing, there is shown adiffusion vacuum pump generally indicated by the reference numeral 1!]which includes a hollow cylindrical casing 12 with an integral bottomwall 14. The upper end of the pump is provided with a suitable inletflange 16 to which a cooperable flange 18 of a vessel 2% to be evacuatedmay be secured. Arranged between the flanges 16 and 18 is a baflle 22which serves to condense the oil vapor which tends to migrate from thepump toward the vessel 20. The discharge side of the pump is providedwith a pipe 24, one end of which opens into the lower portion of tiecasing 12.

With the pump casing there is centrally arranged a hollow nozzleassembly comprising a plurality, and in this case four, jet nozzlesindicated at 26, 28, 3t 32. Vapors rising in the interior of the nozzleassembly due to vaporizaticn of pump fluid are directed out of therespective orifices in the downwardly directed jet nozzles. A suit-3,@75,fi9 Eatented Jan. 29, 1963 able heater 34 located within thenozzle assembly and arranged on the bottom wall 14 serves to heat pumpfluid to cause vaporization thereof.

In FIGURE 2, the jet nozzle 26 is illustrated as comprising a lipportion 36 and a frusto-conical skirt 38 spaced slightly radiallytherefrom. The illustrated jet nozzle arrangement is conventional and,as known in the art, hot oil vapors are driven upwardly through thechimney 40 of the nozzle assembly and expelled at great speed out of theannular spacing 42 between the lip 36 and the skirt 38. Normally thegreater portion of these vapors are directed downwardly through the pumphousing in order to achieve pumping action, however, a small amount ofthe vapors turn the lower edge of the skirt and migrate upwardly towardthe handle 22. tory oil vapor is condensed by the baflfle and oildroplets, resulting therefrom; will drop down into the heater portion ofthe pump to be vaporized. i

. In order to minimize backstreaming or the migration of the oil vaporin the direction of the intake end of the pump housing, the top jetnozzle 26 is provided with an insulator cap 44 which conforms generallyto the frustoconical shape of the skirt 38 but overlaps the lower edgethereof for a considerable distance. The lower edge of the cap extendstoward the fringes of the jet'stream emanating from the jet nozzle andserves to deflect the vapors which turn outwards around the-lower edgeof the skirt 38 and would normally drift upwardly toward the flange 16.These migratory vapors which impinge upon the cap 44- heat the samesomewhat and, in order to prevent re-evaporation of the oils which maycondense on the cap, the same is maintained in a condition wherere-evaporation cannot occur.

In the present invention, this is accomplished by preparing the cap 44-as an insulated body and to this end, the inner or under-surface 48 ismade reflective and a heat shield '50 is provided between the cap andthe skirt of the nozzle 26. Both the upper and the lower surfaces of theshield 5t! are reflective. The reflective under-surface 48 of the capand the reflective lower surface of the shield heat and tend to minimizethe heat that would develop on the cap by hot vapors emanating from thejet nozzle 26. This reduction in the amount of heat reaching the cap 44permits the natural loss of heat through radiation from the jetstructure to the walls of the pump casing to maintain the temperature ofthe cap at a sulficiently low point to prevent re-evaporation of theconensed vapor.

These surfaces also reflect heat back into the jet nozzle causing it tooperate at a somewhat higher temperature, thereby making the streamdrier or free of liquid pump fluid, thus improving the quality of thejet stream expelled from the nozzle throat. In eilect then the cap 44serves to deflect oil vapor which would deviate from the main jet streamof the pump and, to prevent the cap from becoming another or secondarysource of oil vapor, the cap 44 is effectively maintained at atemperature where re-evaporation does not occur by the provision of thelower reflective surfaces of the cap and the shield. With thisarrangement, the need for extraneous cooling devices such as coolingcoils, conduction cooling members, Peltier couples and the like forlarge scale cooling is eliminated.

Deflection of the oil vapors by the shield 44- tends to reshape theotherwise useless fringe portion of the jet stream by directing thesevapors back into the jet stream where they can perform useful pumpingaction. In this manner, the speed of the pump will be increased somewhatover and above a nozzle arrangement without the The reflectivecharacteristics of the lower-surface of This migracap 44 in conjunctionwith the lower reflective surface of the shield 50 produces the netresult of retaining the maximum amount of heat possible within the jetnozzle 26. These characteristics of both surfaces also reduces theamount of heat reaching the cap 44 to a degree that the normal heat lossby radiation to the walls of the pump casing is suflicient to maintainthe temperature of the cap below that required for re-evaporation of thecondensed vapor.

Mounting and separation of the cap 44 and the intermediate shield 50relative to the other structure of the jet nozzle 26 may be accomplishedby any suitable means which will prevent or minimize heat conductionfrom the relatively hot structural elements of the nozzle 26. As shownin FIGURE 2, a suitable mounting means is illustrated and comprises avertically projecting screw 52 secured to and extending from the centerpoint of the skirt 38. The screw 52 projects through suitable centrallylocated apertures formed in the cap 44- and the shield 50. A spacer 54is arranged on the screw between the shield 50 and the cap to positionthe former relative to the latter. A lock nut 56 maintains theaforementioned structure upon the jet nozzle 26. In order to maintain apredetermined spacing between the shield and the skirt 38, there isprovided a plurality of screws 60 (only one shown in FIGURE 2)threadedly secured to the cap and projecting inwardly therefrom andthrough suitable apertures formed in the shield 50. A pair of lock nuts62 received on the screw 60 and positioned on either side of the shieldmaintains the same in a fixed spaced distance from the cap. The innerend of the screw 60 abuts the skirt 38 and is utilized in conjunctionwith the screw 52 to space the cap and shield from the skirt and also tosecure this structure to the skirt.

The present invention illustrated in FIGURE 2 may also be adapted forthe other stages of the diflusion pump illustrated in FIGURE 3. As shownin FIGURE 3, the jet nozzle 28, which comprises the conventional lipportion 64 and skirt 66, is also provided with an insulator cap 68. Theunder-surface of the cap 68 is made reflective and, mounted below thecap 68, is an intermediate shield 70 which is made reflective on bothsurfaces. The inner edge of the cap 68 is radially spaced relative tothe column or stack 40 by suitable long rivets 69, and a screw 71 andmounting devices 72, 74 arranged on'the cap, serve to support the shield70 in spaced relationship relative to the cap and the skirt 66.

The present invention is distinguished from the invention disclosed andclaimed in the copending patent application Serial No. 153,543, filedNovember 20, 1961, assigned to the same assignee in that the presentinvention requires an insulated cap formed with a straight conicaldesign and at least one heat reflective shield. Since in the presentinvention the conical cap is bombarded with vapor molecules at a moreglancing angle, the amount of heat produced on the conical cap by thecondensed vapor is relatively small or at least smaller than if thelower edge of the cap was formed with a generally vertical extendingcylindrical projection shape of the type disclosed in the above referredto copending application. With a straight conical shape cap, as providedin the present invention, the vapor molecules strike the inner :surfaceof the cap at a more glancing angle resulting in .a lesser amount ofcondensed vapor and a correspondingly lesser amount of heat transferredto the cap. On :the other hand, in order to project into the fringeportion of the jet stream, the straight conical cap must be extendedfurther than the cylindrical projection of the invention disclosed inthe copending patent application re sulting in the use of a largerdiameter at the base of the cap of straight conical design. This willlessen the pumping speed of the diffusion pump as compared to the speedwhen using the cap having the design disclosed in the .copending patentapplication. Nevertheless, use of a larger diameter cap of the straightconical design of the present invention does eliminate morebackstreaming than is the case with the small diameter cylindricalprojection design as disclosed in the copending application.

In order to insure further maintaining the cap 44 in a condition toprevent re-evaporation of condensed vapor, the upper surface of the capmay be coated with a material having a higher emissivity than the normalmetallic surface of the cap. An example of such a high emissivitymaterial for use with a cap made from aluminum would be a black dyedanodized surface.

From the foregoing it will be apparent that the present invention may beapplied to any or all of the jet nozzle of the nozzle assembly of adiffusion pump. The provision of an insulated cap deflects oil vaporthat would otherwise migrate upstream of the pump and will permit theuse of a baffle 22 having a greater conductance. Maintaining the cap ina condition for preventing re-evaporation of oil thereon is attained bythe ability of the undersurface of the cap to reflect the heat, theprovision of a reflective surface on the shield and the transfer of heaton the cap to the pump walls by normal radiation. In the use of thepresent invention, there is no need to employ extraneous cooling devicesto effect cooling of the caps 44 and 68.

I claim:

1. In a vapor vacuum pump, a pump casing having an inlet, an outlet, anda boiler for heating pump fluid, a nozzle assembly mounted in the .pumpcasing for providing a vapor jet stream, said nozzle assembly having atleast one jet nozzle comprising a lip portion and a skirt between Whichthe jet of vapor is expelled for pumping action, a cap surrounding saidjet nozzle in spaced relation to said skirt and having an edge extendingbelow said skirt and into the fringe portions of the jet stream, saidcap having the surface which is adjacent the jet nozzle reflectivethereby to reflect some of the heat emanating from the jet nozzle whichwould thermally affect said cap, and a shield mounted between said skirtand said cap in spaced relation therebetween for further reducing thetransfer heat to said cap.

2. In a vapor vacuum pump, a pump casing having an inlet, an outlet, anda boiler for heating pump fluid, nozzle assembly mounted in the pumpcasing for producing a vapor jet stream, a said nozzle assembly havingat least one jet nozzle comprising a lip portion and a frusto-conicalshaped skirt between which the jet of vapor is expelled for pumpingaction, a radiation cap surrounding said jet nozzle in spaced relationto said skirt and having a shape similar to said skirt, said cap havingan edge extending below said skirt and having the surface which isadjacent the jet nozzle reflective thereby to reflect heat emanatingfrom the jet nozzle which would normally offset said cap and a shieldmounted in spaced relation between said skirt and said cap for furtherreducing the transfer of heat to said cap.

3. In a vapor vacuum pump, a pump casing having an inlet, an outlet, anda boiler for heating pump fluid, a nozzle assembly mounted in the pumpcasing for providing a vapor jet stream, said nozzle assembly having atleast one jet nozzle comprising a lip portion and a skirt between whichthe jet of vapor is expelled for pumping action, a cap surrounding saidjet nozzle in spaced relation to said skirt and having an edge extendingbelow said skirt and into the fringe portions of the jet stream, saidcap having the surface which is away from said jet nozzle coated with amaterial of higher emissivity than the material of said cap, and areflective shield interposed between and spaced from said cap and saidskirt for reflecting some of the heat developed by said skirt.

4. In a vapor vacuum pump, a pump casing having an inlet, an outlet, anda boiler for heating pump fluid, a nozzle assembly mounted in the pumpcasing for providing a vapor jet stream, said nozzle assembly having atleast one jet nozzle comprising a lip portion and a skirt between whichthe jet of vapor is expelled for pumping action, a cap surrounding saidjet nozzle in spaced relation to said skirt and having an edge extendingbelow said skirt and into the fringe protions of the jet stream, saidcap having the surface which is adjacent the jet nozzle reflectivethereby to reflect some of the heat emanating from the jet nozzle whichwould thermally afiect said cap,

and a shield interposed between and spaced from said cap and said skirtand having its surface adjacent the skirt made reflective for reflectingsome of the heat developed by said skirt.

No references cited.

1. IN A VAPOR VACUUM PUMP, A PUMP CASING HAVING AN INLET, AN OUTLET, ANDA BOILER FOR HEATING PUMP FLUID, A NOZZLE ASSEMBLY MOUNTED IN THE PUMPCASING FOR PROVIDING A VAPOR JET STREAM, SAID NOZZLE ASSEMBLY HAVING ATLEAST ONE JET NOZZLE COMPRISING A LIP PORTION AND A SKIRT BETWEEN WHICHTHE JET OF VAPOR IS EXPELLED FOR PUMPING ACTION, A CAP SURROUNDING SAIDJET NOZZLE IN SPACED RELATION TO SAID SKIRT AND HAVING AN EDGE EXTENDINGBELOW SAID SKIRT AND INTO THE FRINGE PORTIONS OF THE JET STREAM, SAIDCAP HAVING THE SURFACE WHICH IS ADJACENT THE JET NOZZLE REFLECTIVETHEREBY TO REFLECT SOME OF THE HEAT EMANATING FROM THE JET NOZZLE WHICHWOULD THERMALLY AFFECT SAID CAP, AND A SHIELD MOUNTED BETWEEN SAID SKIRTAND SAID CAP IN SPACED RELATION THEREBETWEEN FOR FURTHER REDUCING THETRANSFER HEAT TO SAID CAP.